1. Field of the Invention
This invention relates to a process for the production of hydrocarbons having a high quality and a boiling point range of gasoline from mixed gases of carbon monoxide and hydrogen (which will hereinafter be referred to as "synthesis gas") and a catalyst suitable for use in this conversion reaction.
2. Description of the Prior Art
The surroundings of petroleum playing the leading part of energy at present are very unstable and it has been feared that a "valley" of energy will come in the last half of 1980s to the 1990s due to deficiency of petroleum. To this end, it is required to practice economy in the consumption of petroleum in future and to use an alternative energy for petroleum such as coal, nuclear fuel, LNG, etc. In particular, it has lately been watched with keen interest to develop the technique of C.sub.1 -chemistry to make up for the short supply of gasoline, kerosene and gas oil which demands will relatively be expanded by producing from other carbon sources than petroleum, e.g. coal and natural gas which can be found in abundance in the world.
Methods of producing hydrocarbons from coal can be classified mainly into two methods: direct method by liquefaction of coal and indirect method through the synthesis gas, and a number or studies have hitherto been made concerning these two methods. The liquefaction of coal is generally carried out by subjecting coal to hydrogenation under a high pressure in the presence of a solvent to obtain gaseous or liquid hydrocarbons, but this method is still under development and unfavourable economically and the quality of the products is inferior to petroleum at present. On the other hand, the indirect method, which has already been put to practical use by SASOL in South Africa, consists in a method of converting a carbon source into hydrocarbons by making carbon monoxide and hydrogen in the presence of air, oxygen or steam and reacting in the presence of a Fischer-Tropsch catalyst. It is well known in the art that carbon sources such as coal, natural gas and asphalt which are hard to be converted directly into gasoline, kerosene or gas oil are converted into mixed gases of carbon monoxide and hydrogen by the gasification technique established as an industrial technique and that the thus resulting mixed gases are contacted in the presence of a suitable catalyst and converted into hydrocarbons. This has been conducted on a commercial basis, as set forth above.
For example, the Fischer-Tropsch process is known as a process for producing hydrocarbon mixtures from the synthesis gas in the presence of a catalyst based on iron, cobalt, nickel, ruthenium, thorium and rhodium. However, the use of this catalyst results in reaction products of hydrocarbons including paraffins and olefins, distributed widely from methane to wax, and of various oxygen-containing compounds including alcohols and ethers and thus it is impossible to obtain selectively valuable products with a specified boiling point range. That is, the yield of the most valuable gasoline fraction is not sufficient and the gasoline fraction is not usable as motor gasoline as it is and should be modified, for example, by catalytic reforming, since it contains little aromatic hydrocarbons or highly branched parrafins or olefins and has low octane number.
Iron catalysts used on a commercial scale as a Fischer-Tropsch catalyst comprise a precipitated catalyst and fused catalyst, to which copper or potassium is added to raise the selectively thereof. These catalysts are effective for increasing waxes in the product, but do not serve to increase the yield of gasoline fraction and the octane number. On the other hand, ruthenium catalysts are excellent in the formation of high molecular weight waxes, but give only a low conversion of carbon monoxide unless the reaction pressure is kept more than 50 kg/cm.sup.2 and produce liquid hydrocarbons enriched with n-paraffins whose estimation as gasoline is low. In addition, rhodium is known as a noble metal effective for the Fischer-Tropsch synthesis, but it results in a product consisting predominantly of oxygen-containing compounds in spite of its high activity. Other noble metals such as platinum, palladium and iridium have scarcely a catalytic activity according to some reports. Nickel is a methanation catalyst rather than the Fischer-Tropsch catalyst, since it has a very high conversion activity of carbon monoxide, but the resulting hydrocarbon is substantially methane.
Moreover, a two-stage process is known wherein the synthesis gas is contacted with a carbon monoxide reducing catalyst and the product is then contacted with a high silica zeolite catalyst of specified type charged in a same or different reactor, thus converting the synthesis gas into hydrocarbons containing mainly gasoline fraction with high octane number. The carbon nomoxide reducing catalyst used herein is a methanol synthesis catalyst containing two or more metals of copper, zinc and chromium or a Fischer-Tropsch synthesis catalyst of iron type consisting of precipitated iron or fused iron. The two-stage conversion process consists in producing gasoline fraction having high octane number in a high yield by converting once the synthesis gas into oxygen-containing compounds in the case of the methanol synthesis catalyst or converting the synthesis gas into hydrocarbons distributed widely from methane to waxes and oxygen-containing compounds in the case of the Fischer-Tropsch synthesis catalyst, and thereafter, contacting these products with a zeolite catalyst having a specified pore diameter.
Of late, processes for producing selectively hydrocarbons with a specified boiling point range from the synthesis gas by one stage have been found, one of which consists in using a catalyst obtained by mixing mechanically the carbon monoxide reducing catalyst and a specified zeolite used in the two-stage process (U.S. Pat. No. 4,086,262), and the other of which consists in using a catalyst obtained by supporting a carbon monoxide reducing metal or metal oxide on a specified zeolite (U.S. Pat. No. 4,157,338). In any process, the product is limited by the shape selectivity of a zeolite with specified pores as a consititutional component of the catalyst so that products having a larger molecular size than the pore diameter are hardly formed and hydrocarbons having a smaller molecular size and boiling point range of gasoline or less can selectively be obtained. The one-stage process is a more economical process than the two-stage process because of its simplified process. However, the above described one-stage process using the mechanically mixed catalyst is inferior to the two-stage process because of the catalytic defects that the conversion of carbon monoxide and the yield of gasoline are low and there is formed a large amount of methane which is only estimated as a fuel gas. The other one-stage process using the catalyst obtained by supporting a metal capable of exhibiting a Fischer-Tropsch activity on a particular zeolite aims at subjecting the synthesis gas to a Fischer-Tropsch reaction by the metallic component in the catalytic composition to form a hydrocarbon mixture distributed from methane to waxes as an intermediate and then converting these hydrocarbons into hydrocarbons having a boiling point range of gasoline or less by the shape selectivity zeolite known to be effective for cracking waxes, i.e. ZSM-5 zeolite catalyst. The catalyst of this kind is prepared by impregnating ZSM-5 zeolite with iron or ruthenium as disclosed in Japanese Patent Application (OPI) No. 142502/1975. The former catalyst gives a relatively high conversion, but has the drawbacks that the conversion of carbon monoxide to carbon dioxide is increased resulting in decrease of the yield of C.sub.5+ gasoline fraction and the resulting hydrocarbons consist predominantly of methane, while the latter catalyst gives a higher yield of C.sub.5+ gasoline fraction than the former catalyst, but has the drawbacks on practical use that the activity is rapidly deteriorated with the passage of time and a higher reaction pressure, e.g. more than 50 Kg/cm.sup.2 is required in order to obtain C.sub.5+ gasoline fraction effectively. This catalyst is not useful because is loses the activity through heating at a temperature above 300.degree. C. in an oxidizing atmosphere and thus it cannot be regenerated.